It is a common practice to sustain the life of cut plants such as flowers by means of inserting a basal end of the plant stem in a reservoir containing a liquid such as water, or a nutrient liquid. Such a method provides some increase in the life of a plant or flower, but for many varietals the flower lasts but one or two days prior to the onset of wilting even when the cut stem of the flower is immersed in a liquid-containing reservoir such as a vase. It should be understood that the term liquid as used herein is meant to include water, a water solution containing plant nutrients, or any other solution in which a plant stem is immersed to sustain the plant.
Accordingly, it has been a desideratum to develop a method or device for increasing the life of flowers or other plants which are cut from their natural state. For example, U.S. Pat. No. 3,842,539 discloses a device which inhibits the flow of water to a stem of a cut flower, and contains ion exchange resin to remove harmful ions from the water. The method involves the placing of the basal end of the flower stem in a conduit or tube, and the water enters the basal end of the stem only after passage through the resin mass. U.S. Pat. No. 4,103,457 discloses a totally enclosed nutrient liquid supply system including a pouch, tubing and latex sleeve around the stem of the flower, with pressure on the pouch feeding liquid to the stem. The enclosure of the liquid supply system is said to inhibit contamination of the nutrient liquid during protracted periods. The disclosures of U.S. Pat. Nos. 3,842,539 and 4,103,457 are both incorporated herein by reference.
While these two patents have attempted to preserve the life of cut flowers, the methods are significantly complex and costly, and even so fail to provide a practical or successful method or device for preserving the life of a cut plant or flower.
The present invention provides a novel system for providing liquid to a cut plant wherein substantially all the liquid required by the plant is presented to a basal end of the plant stem only after passage through an apparatus which includes a filtration means having a porosity which is sufficient to permit liquid to pass from the reservoir to the stem, but insufficient to permit the passage of microorganisms with the liquid.
In a preferred embodiment, all liquid required by the plant passes through a porous membrane having a porosity which is insufficient to pass microorganisms. As used herein, the term microorganism is meant to include all organisms such as bacteria, fungi and viruses which are harmful to plant survival (i.e., pathogenic organisms) or microscopic or submicroscopic organisms which grow on a nutrient substrate such as plant tissue. While I do not wish to be bound to any particular theory, it appears that plant wilting, when the cut stem of a plant is immersed in a nutrient liquid or water, is due primarily to the growth of such microorganisms on the exposed xylem tissue of the cut stem, which blocks the flow of liquid into the xylem tissue or, in the case of pathogenic microorganisms, may tend to poison the plant. When the plant's vascular system is thus blocked, the negative pressure on the vascular system from transpiration from the plant leaves causes cavitation, which results in the collapse of the vascular system and wilting of the plant.
Broadly, one aspect of the invention comprises providing a conduit or tube having an end adapted to receive a basal end of the plant stem, and including a means to seal a stem receiving portion of the apparatus and the basal end therewithin from the influx of microorganisms while permitting the transport of water or a nutrient medium. Preferably, this stem receiving portion of the apparatus is sterilized prior to use to assist in maintaining the basal end of the isolated from microorganisms.
A portion of the conduit is adapted to be exposed to the liquid in the reservoir. A filtration means such as a membrane or a filtering portion of the apparatus is disposed between the stem receiving portion and the liquid in the reservoir in a manner such that all liquid which passes to the plant from the reservoir must flow through this filtration means, having a porosity which is insufficient to permit the passage of microorganisms, so that the described blockage of the liquid flow is avoided.
Generally, a filter having an effective porosity of less than 3 microns, preferably from about 0.05 to about 0.45 microns, is sufficient to permit the passage of liquid such as water to the stem, but insufficient to permit the passage of deleterious microorganisms with the liquid. Most preferably, a filter having an effective porosity of about 0.2 microns or less is employed. Effective porosity, as used herein, is defined as the ability of the filter to resist the transfer of particles of the specified size.
The hydrophilic filters of the invention, that is, filters which pass liquid but are capable of excluding the passage of microorganisms to the stem, also serve to inhibit the passage of air bubbles (emboli) which might otherwise enter and "air-lock" the plant's vascular system, blocking the uptake of liquid and contributing to wilting, while permitting the passage of dissolved gases necessary for sustaining the plant.
As shown in the embodiments described herein, the filtration means may be a planar film having the appropriate porosity, or may comprise a porous mesh-like or woven polymer which acts as a depth filter, that is, a polymer which has an effective porosity sufficiently small to resist the passage of microorganisms even though the actual porosity of a thin layer of the material is larger than the required amount. The filtration means may also be a hollow fiber having a lumen with a diameter which is sufficient to permit the fiber to wick the fluid from the reservoir to the stem, if no liquid pressure is applied, and a wall porosity which is sufficient to permit the transport of liquid but insufficient to permit the passage of microorganisms with the liquid.
The hollow fiber may have a wall porosity of less than 3 microns, preferably from about 0.05 to about 0.45 microns, and most preferably having a porosity defined by having pores of less than about 0.2 microns diameter. Such fibers may be made, as is known in the art, from a hydrophilic resinous material. Since the basal end of the plant stem is sealed within a first portion of the conduit, flow through this filtration means is the only source of liquid for the plant and the plant accordingly receives fluid from the reservoir in the absence of deleterious microorganisms.
In another aspect of the invention, the sterile conduit portion includes a bactericidal means disposed to be adjacent the stem to further protect the cut end of the stem from the influx of microorganisms which contributes to vascular blockage.
Another contributing cause of wilting in cut flowers and plants is improper liquid pressure on the plant's vascular system. The transport of water in a natural plant is the result of a combination of "pull" from the evaporation of water from the leaves and "push" from pressure created by the roots. When a plant is harvested the positive pressure on the vascular system from the roots is removed, and if the plant stem is of sufficient height it appears that cavitation will occur, that is, the formation and collapse of regions of low pressure, and massive tissue damage ensues which promotes the death of the cutting.
According to another aspect of the invention, significant additional advantages with respect to longer stemmed plants are provided by the use of a pressure system which produces a pressure of from about 0.9 to about 1.2 psi to the liquid at the cut end of the plant stem. Broadly, a method for preserving a plant having a stem, wherein the stem is supplied from a liquid source with a liquid is provided which includes placing a basal end of the stem in an apparatus having a conduit with sealing means disposed around the stem which seal the conduit portion and the basal end therewithin against the influx of microorganisms, and providing a pressure of from about 0.9 to about 1.2 psi on the liquid at cut end of the stem. Preferably, the liquid in the liquid source is sterile.
Significant advantages result from the combination of the filtration device and the pressure system. Preferably, a device for the display of a plant having a stem with a basal portion including a cut end is provided which comprises a first portion adapted to contain liquid; a second portion defining an area adapted to receive the cut end of the stem, and to receive liquid from the first portion; and a sealing means on the second portion, adapted to enclose the basal portion of the stem and seal the stem against the egress of liquid from the device. The display device also includes a filter having a porosity of from about 0.05 to about 0.45 microns disposed between the first and the second portions and disposed to seal the second portion from the first portion in a manner such that microorganisms cannot flow from the first to the second portion, and means for providing a pressure of from about 0.9 to about 1.2 psi on the liquid at cut end of the stem.
Since long stemmed plants also benefit from a positive pressure system during transport, a novel method for the transport of plants is provided wherein the filtration device of the invention is filled with water and attached to the cut end of the plant, and the plant is then positioned with the leaves or flower lower than the cut stem, so that the combined stem and device act as a water column to provide sufficient pressure to overcome the tendency of the vascular system to cavitate.